Sports betting question, co-related parlays

[youtalkfunny]

I'm no math wiz, just a former bookie, and I got bogged down in the numbers when trying to explain a simple concept, and I can't make the numbers work the way I thought they should.

QUESTION: in the game cited below, determine the probability of four possible outcomes:

Fav and over = ?%
Fav and under= ?%
Dog and over = ?%
Dog and under= ?%

Given:

--the pointspread is an accurate equalizer. Favorites cover 50%, dogs cover 50%, over 50%, under 50%.
--other than that, we're in the real world

So if we're betting 2-teamers, we can presume that, all things being equal, all four possible outcomes:

fav and over
fav and under
dog and over
dog and under

each come in 25% of the time.

Now let's take an example where all things are NOT equal

Fav State vs Dog U

Fav -30, over/under on the game is 33.

Let's say we throw in another given: In this particular game, with these lines, the chance of fav and under coming in isn't any where near 25%, but we'll call it 2%.

Can we determine how often the other three possible outcomes should be expected?

Or is it too unreal an example to cling to the notion that the lines make each proposition 50-50?